The present invention relates to a memory device. More particularly, the present invention relates to a semiconductor device having a modified device isolation structure with improved carrier mobility and a method for its fabrication.
Generally, integrated circuit (“IC”) makers have reduced the size of device in order to increase operating speed of the device such as transistors constituting IC. Although the shrunk device may be operated high-speedily, there is an adverse effect to the transistor's performance, which is called to the Short Channel Effect (“SCE”) such as the decrease in breakdown voltage of source/drain regions, the increase in junction capacitance, and the instability of critical voltage.
The technique of increasing operating speed of the device has changed from reducing the size of the device to improving the carrier mobility of a transistor and reducing the SCE of the device. The carrier mobility of the transistor can be improved by straining the semiconductor devices. When stress is applied to a transistor in order to improve the operating characteristics of an n-type Metal Oxide Semiconductor (“NMOS”) transistor and a p-type Metal Oxide Semiconductor (“PMOS”) transistor, tensile stress and compressive stress are applied along the channel direction of the device, respectively. First, there was an attempt to improve the carrier mobility by applying different stress according to kind of transistor. That is, the stress can be adjusted according to the deposition material and deposition condition for the gate spacer in the formation of the gate spacer. However, since the gate material has changed from a ‘hard’ tungsten silicide layer to a ‘soft’ tungsten layer, the gate material plays a role of a buffer against stress applied to the gate structure. As a result, it is difficult to apply the desired stress to the transistor. In addition, there is a huge cost to improve the carrier mobility of the device by using silicon germanium source/drain regions or using a Silicon-on-Insulator (“SOI”) substrate.